1. Field of the Invention
The present invention relates to a position measurement apparatus which measures a position of an object such as a mask or a wafer with high accuracy.
2. Description of the Related Art
Recently, a significant progress in a microfabrication technology is seen. When exposure light is changed to short wavelength light such as that from a KrF excimer laser, a glass material that is to be used is limited, and it is difficult to correct a wavelength difference with respect to an alignment wavelength of a projection optical system. Therefore, in an exposure apparatus, adopting an off-axis alignment detection system (an OA detection system) that is not affected by the influence of the wavelength difference is required. In addition, a highly accurate focus system that measures a surface position of a wafer is required.
Conventionally, in a wafer alignment system or a focus system, in order to reduce a position measurement error, a reference optical system is configured so as to perform an absolute value correction measurement. Japanese Patent Laid-Open No. H2-206706 discloses a configuration of including an optical system of measurement reference light separately from an optical system of measuring a wafer in the correction measurement of the wafer alignment system. Japanese Patent Laid-Open No. 2006-337373 discloses a configuration of including an optical system of measurement reference light separately from an optical system of measuring a wafer in the correction measurement of the focus system. In the configuration disclosed in Japanese Patent Laid-Open Nos. H2-206706 and 2006-337373, when the wavelengths of the reference light and the measurement light are different from each other, a wavelength difference is generated in a detection system. Since this wavelength difference causes a position displacement on a sensor, an error is generated in a measurement value. Therefore, in Japanese Patent Laid-Open No. 2002-164268, a configuration of correcting the position displacement caused by such a wavelength difference is disclosed.
A wavelength of the measurement light that is used in a position measurement apparatus is variable, and on the other hand, a wavelength of the reference light is fixed. Therefore, the correction of the measurement value is performed by adding or subtracting a predetermined correction value (an offset value) for an actual measurement value. However, strictly, a correction amount that is required for the actual measurement value is different in accordance with a difference of the wavelength of the measurement light and the wavelength of the reference light. Therefore, considering only a constant correction amount, an exact correction cannot be performed. Furthermore, in an actual position measurement apparatus, in order to improve the ability to handle a manufacturing error of a component part such as an optical part or an assembly error, a use wavelength band tends to be widened.